1. Field of the Invention
The present invention relates to a high zoom-ratio zoom lens system having a zoom ratio exceeding 13, which is suitable for an SLR camera, especially for a digital SLR camera.
2. Description of Related Art
In a high zoom-ratio zoom lens system having a zoom ratio of approximately 10, a zoom lens system of four lens groups, e.g., a lens group having a positive refractive power (a positive lens group), a lens group having a negative refractive power (a negative lens group), and other two positive lens groups in this order from the object, is commonly used (e.g., Japanese Unexamined Patent Publication Nos. 2008-003195, 2006-259016 and 2003-241097).
Furthermore, a zoom lens system of five lens groups (e.g., Japanese Unexamined Patent Publication Nos. H10-333037 and 2006-337648), and a zoom lens system of six lens groups (Japanese Unexamined Patent Publication No. H04-186212) are also known in the art.
In the high zoom-ratio zoom lens system having the four lens groups such as in JUPP Nos. 2008-003195, 2006-259016 and 2003-241097, the focal length at the long focal length extremity is not sufficiently secured, i.e., around 250 mm to 300 mm; and, since the second lens group mainly carrying out zooming is also arranged to function as a focusing lens group, the second lens group is required to have four to six lens elements. Therefore such a zoom lens system is not suitable for autofocusing since the focusing lens group (i.e., the second lens group) becomes heavier. Furthermore, the F-number is approximately 6.3, which means that the illumination in an image is dark.
In JUPP No. H10-333037, a fifth lens group having a weak negative refractive power is added to a zoom lens system of the four lens groups, so that the fundamental characteristics thereof are same as those of JUPP Nos. 2008-003195, 2006-259016 and 2003-241097.
The zoom lens system disclosed in JUPP No. 2006-337648 is the so-called ‘telephoto’ zoom lens system in which the angle-of-view at the short focal length extremity is less than 35°.
The zoom lens system disclosed in JUPP No. H04-186212, in which optical performance at the telephoto side is particularly improved, achieves a focal length of 350 mm at the long focal length extremity and an F-number of 5.6; however, the focal length at the short focal length extremity is 35 mm. If such a zoom lens system is used in an APS-C sized digital SLR camera, the angle-of-view at the short focal length extremity is only around 45°.